Thermoformable silicone resin coating composition and dual component coating system for polycarbonate

ABSTRACT

Ultraviolet radiation resistant silicone resin coating compositions are provided having improved thermoformability. Improved thermoformability and shortened required aging are achieved by the addition of a Lewis acid compound to the coating compositions is disclosed. Thermoformability is also improved by using the UV screen containing silicone resin coating composition in conjunction with extensible acrylate primers.

BACKGROUND OF THE INVENTION

This invention relates to thermoplastic substrates covered with aprotective coating. More particularly, it relates to improved siliconeresin coating compositions for substrates which, after curing, may bethermoformed without showing cracks or adhesion failure. Thethermoformable top coat forms an adherent protective, abrasion-resistantlayer on the substrate.

Recently, the substitution of glass glazing with transparent materialswhich do not shatter or are more resistant to shattering than glass hasbecome widespread. For example, transparent glazing made from syntheticorganic polymers is now utilized in public transportation vehicles suchas trains, buses, taxis and airplanes. Lenses for eye glasses and otheroptical instruments, as well as glazing for large buildings, also employshatter-resistant, transparent plastics. The lighter weight of theseplastics in comparison to glass is a further advantage, especially inthe transportation industry where the weight of the vehicle is a majorfactor in its fuel economy.

While transparent plastics provide the major advantage of being moreresistant to shattering and lighter than glass, a serious drawback liesin the ease with which these plastics mar and scratch due to everydaycontact with abrasives such as dust or cleaning equipment. Marringresults in impaired visibility and poor aesthetics, and often requiresreplacement of the glazing or lens or the like.

One of the most promising and widely used transparent plastics forglazing is polycarbonate, such as that known as Lexan®, sold by GeneralElectric Company. It is a tough material, having high impact strength,high heat deflection temperature and good dimensional stability. It isalso self-extinguishing and easily fabricated. Acrylics, such aspolymethylmethacrylate, are also widely used transparent plastics forglazing.

To improve the abrasion resistance of plastics, mar-resistant coatingsformed from mixtures of silica, such as colloidal silica or silica gel,and hydrolyzable silanes in a hydrolysis medium, such as alcohol andwater, have been developed. U.S. Pat. Nos. 3,708,225 (Misch et al.),3,986,997 (Clark), 4,027,073 (Clark), 4,159,206 (Armbruster et al.) and4,177,315 (Ubersax), for example, describe such compositions. Improvedsuch compositions are described in commonly assigned copending U.S.Application Ser. No. 964,910, filed Nov. 30, 1978 (now abandoned) andU.S. Pat. Nos. 4,277,287 (Frye) and 4,309,319 (Vaughn, Jr.).

Silicone resin coatings have been additionally improved in recent yearsby the addition of compounds which screen or absorb ultravioletradiation and thereby serve to prolong the service life of coatedarticles. Such coatings and compounds are described, for example, inU.S. Pat. Nos. 4,299,746 (Frye) and 4,278,804 (Ashby et al.), 4,373,060(Ching), 4,373,061 (Ching), and 4,374,674 (Ashby et al.)

In order to promote adhesion of silicone resin coating compositions tothermoplastic substrates such as Lexan® or polymethylmethacrylate,primer coats are often necessary. U.S. Pat. No. 4,188,451 (Humphrey),for example, discloses the UV-cured reaction product of a polyfunctionalacrylic ester and an unsaturated radical-containing silane as a primer,and the aforementioned U.S. Pat. No. 4,309,319 discloses the use of athermosetting acrylic resin. The aforementioned Ching patents, on theother hand, disclose UV screen-functionalized compositions which do notneed primers and instead adhere directly to polycarbonate substrateswithout pre-treatment (other than routine cleaning), although an agingperiod is required after preparation of the compositions before they areready to use.

All of the above-mentioned patents and applications are incorporatedherein by reference.

Two disadvantages to the abrasion-resistant silicone resin coatingcompositions currently used to coat polycarbonate are that they are notthermoformable after curing, especially in thick applications, andprimerless coatings must be aged, as mentioned previously. Poorthermoformability means that bending or working a coated polycarbonatesubstrate will often lead to cracking or crazing of the siliconecoating. As a consequence, articles must be coated after forming, whichmay entail time delays and shipment of uncoated articles which may beinadvertently abraded in transit. The aging period required for theaforementioned primerless coating compositions entails warehousing theprepared compositions, the coats of which may outweigh any disadvantagesin using primer coats.

It has now been discovered that the thermoformability of the UVscreen-functionalized compositions described by Ching in U.S. Pat. Nos.4,373,060 and 4,373,061 can be improved and their aging timesubstantially reduced by the addition of a small amount of a Lewis acidcompound, such as ferric chloride, aluminum chloride, zinc chloride, andthe like. Thermoformability and other physical properties are alsoimproved when those coating compositions are used in conjunction withthermoplastic acrylate primers. Use of a Lewis acid compound has beenfound to assist the incorporation of silylated UV screens into thestructure of silicone resin coating compositions and to improve theirthermoformability. Employing extensible acrylate primers with theabove-mentioned silicone resins, which are disclosed to be primerlesscoating compositions on polycarbonate, results in a dual componentcoating system which is thermoformable after curing.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide asilicone resin coating composition with improved thermoformability whenapplied to thermoplastic substrates.

It is a further object of the present invention to provide a means ofreducing the aging period required in certain silicone resin coatingcompositions containing silylated ultraviolet radiation screeningcompounds.

It is a further object of the present invention to provide a dualcomponent silicone resin coating system for polycarbonate which isthermoformable.

These and other objects are accomplished herein by a silicone resincoating composition comprising a colloidal silica filled thermosetorganopolysiloxane containing a silylated ultraviolet radiationscreening compound and a small amount of a Lewis acid compound.

Another embodiment of the present invention provides a dual componentcoating system for thermoplastic substrates which includes:

(A) a primer composition comprising from about 0.5 to about 10 parts byweight of a thermoplastic acrylic polymer in about 99.5 to about 90parts by weight of a volatile solvent, and

(B) a colloidal silica filled thermoset organopolysiloxane coatingcomposition containing a silylated ultraviolet radiation screeningcompound and optionally a small amount of a Lewis acid compound.

Also contemplated herein are thermoformable coated articles comprising apolycarbonate substrate having at least one surface coated with:

(1) a layer of a primer composition comprising from about 0.5 to about10 parts by weight of a thermoplastic acrylic polymer in about 99.5 toabout 90 parts by weight of a volatile solvent, and

(2) a layer of a cured top coat containing a colloidal silica filledthermoset organopolysiloxane containing a silylated ultravioletradiation screening compound and, optionally, a small amount of a Lewisacid compound.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to primerless* silylated UV screen-containingorganopolysiloxane coating compositions for polycarbonate substrateswhich are rendered thermoformable after curing by addition of a Lewisacid compound and/or by using said composition in conjunction with athermoplastic acrylic polymer primer composition. The coatingcompositions of the present invention can be applied to a polycarbonatesubstrate and cured by heating for short periods at temperatures in therange of from about 90° C. to about 200° C., after which the coatedsubstrate may be thermoformed without inducing cracking or crazing inthe cured coating. Similarly, where the dual component coating system ofthe present invention is employed the acrylate primer layer is appliedto a substrate and thermally dried, e.g., at 125° C. for 15-30 minutes,and the organopolysiloxane top coat is applied and cured, after whichthe coated polycarbonate substrate may be thermoformed without crackingthe protective silicone resin coating.

"Thermoforming" is a well known term in the plastics art describing theprocess of shaping thermoplastic sheets by heating them until softened,then forming them into desired shapes on a mold or jig. As used herein,the term extends also to the coatings applied to thermoplastic sheetswhich are later thermoformed. Thermoformable coatings are those whichcan be successfully applied to a sheet substrate, cured and thermoformedalong with the substrate without showing cracking or crazing.

The polycarbonates for which the coating compositions and coating systemof the present invention are especially useful include polycarbonateshaving recurring units of the formula: ##STR1## wherein each --R-- isselected from the group consisting of phenylene, halo-substitutedphenylene and alkyl substituted phenylene; and A and B are each selectedfrom the group consisting of hydrogen, hydrocarbon radicals free fromaliphatic unsaturation and of radicals which together with the adjoiningcarbon atom form a cycloalkane radical, the total number of carbon atomsin A and B being up to 12.

The aromatic carbonate polymer may be prepared by methods well known inthe art and as described in U.S. Pat. No. 3,989,672, all of which areincorporated by reference.

Also included herein are branched polycarbonates wherein apolyfunctional aromatic compound is reacted with the dihydric phenol andcarbonate precursor to provide a thermoplastic randomly branchedpolycarbonate wherein the recurring units of Formula I contain branchinggroups.

The preferred polycarbonate resins may be derived from the reaction ofbisphenol-A and phosgene. These polycarbonates have from 10-400recurring units of the formula: ##STR2##

The polycarbonate should have an intrinsic viscosity between 0.3 and1.0, preferably from 0.40 to 0.65 as measured at 25° C. in methylenechloride.

The thermoplastic acrylic polymers which are employed in the primingmaterial in accordance with this invention are those thermoplasticacrylic polymers well known in the art as thermoplastic acrylicpolymers. Exemplary thermoplastic acrylic polymers employed in thepractice of this invention are set forth, for example, in Encyclopediaof Polymer Science and Technology, Vol. 1, Interscience Publishers, JohnWiley & Sons, Inc., copyright 1964, at pp. 246 et seq. and thereferences cited therein, all of which are hereby incorporated byreference.

The term "thermoplastic acrylic polymers" as used herein is meant toembrace within its scope those thermoplastic polymers resulting from thepolymerization of one or more acrylic acid esters monomers as well asmethacrylic acid ester monomers. These monomers are represented by thegeneral formula:

    CH.sub.2 ═CYCOOR'                                      (III)

wherein Y is H or a methyl radical and R' is an alkyl radical,preferably one containing from 1 to about 20 carbon atoms.

Examples of alkyl groups represented by R' in the above general formulaare methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl,tert-butyl, n-amyl and the various positional isomers thereof, andlikewise the corresponding straight and branched chain isomers ofpentyl, hexyl, heptyl, octyl, nonyl, decyl and the like.

Exemplary acrylic acid ester monomers represented by Formula III includemethyl acrylate, isopropyl acrylate, n-propyl acrylate, n-butylacrylate, isobutyl acrylate, sec-butyl acrylate, 2-ethylhexyl acrylate,etc. Exemplary methacrylic acid ester monomers represented by FormulaIII include methyl methacrylate, ethyl methacrylate, n-butylmethacrylate, isobutyl methacrylate, hexyl methacrylate, etc. Copolymersof the above acrylate and/or methacrylate monomers are also includedwithin the term "thermoplastic acrylic polymers" as it appears herein.The polymerization of the monomeric acrylic acid esters and methacrylicacid esters to provide the thermoplastic acrylic polymers useful in thepractice of the invention may be accomplished by any of the well knownpolymerization techniques. The thermoplastic acrylic polymers having amolecular weight of at least about 15,000 generally are preferred in thepractice of the instant invention.

The thermoplastic acrylic polymers useful in the practice of the instantinvention include acrylic ester homopolymers derived from acrylic acidester monomers; methacrylic ester homopolymers derived from methacrylicacid ester monomers; and copolymers derived from two different acrylicacid ester monomers, or two different methacrylic acid ester monomers,or an acrylic acid ester monomer and a methacrylic acid ester monomer.

Mixtures of two or more of the aforedescribed thermoplastic acrylicpoymers, e.g., two or more different acrylic ester homopolymers, two ormore different acrylic ester copolymers, two or more differentmethacrylic ester homopolymers, two or more different methacrylic estercopolymers, an acrylic ester homopolymer and a methacrylic esterhomopolymer, an acrylic ester copolymer and an acrylic ester copolymer,an acrylic ester homopolymer and a methacrylic ester copolymer, etc.,can also be used in the present invention.

The thermoplastic acrylic polymers are in general applied as primersfrom a primer composition containing said thermoplastic acrylic polymerand a volatile solvent, either organic or inorganic in nature, which isinert, i.e., will not react with the polycarbonate part to be treated,but which is capable of dissolving the thermoplastic acrylic polymers.Generally, the concentration of the thermoplastic acrylic polymer in thepriming composition ranges from about 0.5 to about 10 percent by weight,preferably from about 0.5 to about 2 percent. Examples of suitablesolvent systems include ethylene glycol diacetate, butoxyethanol, ethoxyethanol, diacetone alcohol, hydroxyethyl acetate, cellosolve acetate andcombinations thereof.

The primer compositions thus prepared must be extensible, having adegree of plasticity or softening at thermoforming temperatures, so asnot to crack or craze upon thermoforming.

The primer compositions of the instant invention may also optionallycontain various flatting agents, ultraviolet light absorbing agents,surface active agents and thixotropic agents. All of these additives andthe use thereof are well known in the art and do not require extensivediscussions.

When the dual component coating system of the present invention, whichincludes a primer composition, is employed, a uniform film of the primercomposition is applied onto the polycarbonate surface by any of theknown means such as dipping, spraying, roll-coating and the like. Afterthe polycarbonate substrate is coated with the primer composition, theinert volatile solvent is removed by drying the coated article until thevolatile solvent evaporates leaving a primer layer containing thethermoplastic acrylic polymer on the polycarbonate surface to which theprimer composition was applied. It has been found necessary to obtainthe advantages of the present invention that the primer layer bethermally dried, after application to the polycarbonate substrate, attemperatures from about 90° C. to 130° C. It has been observed thatcoated articles having air dried primer layers do not exhibit the degreeof thermoformability contemplated herein. Generally, the primer layer isa uniform film having a thickness varying between about 0.002 mil toabout 0.1 mil, preferably between about 0.01 mil to about 0.05 mil.

In the dual component coating system of the present invention, after thepolycarbonate has been primed by the application of the primercomposition and the solvents of the primer have been evaporated, theprimed polycarbonate substrate is then coated with the colloidal silicafilled thermoset organopolysiloxane containing a silylated ultravioletradiation screening compound and optionally containing a small amount ofa Lewis acid compound.

For the purposes herein, a "Lewis acid compound" is a substance thatwill act in solution as a Lewis acid, that is, will taken up an electronpair to form a covalent bond. In other words, they are compounds whichact as "electron pair acceptors". This includes the "proton donor"concept of the Lowry-Br nsted definition of acids.

Lewis acid compounds, when added to the silylated UV screen-containingsilica filled polysiloxane coating compositions of the presentinvention, improve the thermoformability of the polysiloxane coatingcompositions and also reduce the aging period often required before theycan be used. Preferred Lewis acid compounds for the purposes hereininclude aluminum chloride, ferric chloride and zinc chloride. Ferricchloride is most preferred.

Very small amounts of the Lewis acid compounds will effect improvementnot only in thermoformability and aging time, but also will improveresistance of the coating to cracking from UV exposure and to loss ofadhesion from moisture. For the purposes of the present invention, fromabout 10 to 30 parts per million (ppm) are preferred, but any amountsufficient to enhance the thermoformability of the cured coatingcompositions or reduce the required aging time is contemplated.

When the aforementioned Lewis acid compounds are employed in thecolloidal silica filled thermoset organopolysiloxane coatingcompositions herein, the coating composition, which contains a silylatedUV screen, may be applied to a polycarbonate substrate without a primercoat, and the coating composition will show improved thermoformability.However, coating compositions treated with Lewis acid compounds asdescribed above are also contemplated for use in the dual componentcoating system of this invention.

Colloidal silica filled thermoset organopolysiloxane coatingcompositions which are suitable for the purposes herein are disclosed inthe patents and applications already mentioned and incorporated byreference above. Generally, they are comprised of a dispersion ofcolloidal silica in a lower aliphatic alcohol-water solution of thepartial condensate of a silanol having the formula:

    R.sup.2 Si(OH).sub.3                                       (IV)

wherein R² is selected from the group consisting of alkyl radicalscontaining from 1 to 3 carbon atoms, vinyl, 3,3,3-trifluoropropylradicals and γ-glycidoxypropyl radicals, with at least 70 percent byweight of said silanol being CH₃ Si(OH)₃. This composition generallycontains from about 10 to about 50 percent by weight of solids, saidsolids consisting essentially of a mixture of from about 10 to about 70percent by weight of colloidal silica and from about 30 to about 90percent by weight of the partial condensate of a silanol. The partialcondensate of a silanol, i.e., a siloxanol, is obtained, preferably,entirely from the condensation of CH₃ Si(OH)₃, however, the partialcondensate may also optionally be comprised of a major portion which isobtained from the condensation of CH₃ Si(OH)₃ and a minor portion whichis obtained from the condensation of monoethyltrisilanol,monopropyltrisilanol, monovinyltrisilanol, monogamma-methacryloxypropyltrisilanol, monogamma-glydicoxypropyltrisilanol, or mixtures thereof.

The trisilanol component of the top coat composition of the presentinvention is generated in situ by the addition of the correspondingtrialkoxysilanes to aqueous dispersions of colloidal silica. Suitabletrialkoxysilanes are those containing methoxy, ethoxy, isopropoxy andt-butoxy substituents. Upon generation of the silanol in the aqueousmedium, there is condensation of the hydroxyl substituents to form--Si--O--Si-- bonding. The condensation is not complete, but rather thesiloxane retains an appreciable quantity of silicon-bonded hydroxylgroups, thus rendering the organopolysiloxane polymer soluble in thewater-alcohol solvent. This soluble partial condensate can becharacterized as a siloxanol polymer having at least one silicon-bondedhydroxyl group per every three --SiO-- units. During cure of the topcoating composition on the primer, these residual hydroxyl groupscondense to give a silsesquioxane, R² SiO_(3/2).

The silica component of the coating composition is present in the formof colloidal silica. Aqueous colloidal silica dispersions varysubstantially in particle size. Typical available commercial silicashave particle sizes in the range of 3 to 100 millimicrons in diameter.Particles up to 150 millimicrons in diameter can be used in thesecoating compositions. These silica dispersions are prepared by methodswell known in the art and are commercially available. It is preferred touse colloidal silica having a particle size in the range of 5 to 20millimicrons in diameter in order to obtain dispersions having a greaterstability and to provide top coatings having superior opticalproperties.

The colloidal silica filled thermoset polysiloxane coating compositionsof the present invention are typically prepared by addingtriakoxysilanes to a colloidal silica hydrosol and adjusting the pH byadding acid or base. Such compositions are disclosed to have a useful pHrange of 3.0 to 8.0, with variations in particular physical properties,such as cure time and abrasion resistance, occurring within that range.Simple experimentation will allow the practicioner to tailor thesilicone resins' characteristics to his individual needs. Acid (or base)may be added either to the silane component or the silica hydrosolcomponent before the two are mixed to adjust the pH.

Alcohol is generated during the hydrolysis of the trialkoxysilanes tothe silanetriols. Depending on the percent solids desired in the finalcoating composition, additional alcohol, water, or a water-misciblesolvent may be added. Suitable alcohols for this purpose are loweralkphatic alcohols such as methanol, ethanol, isopropanol, t-butanol,isobutanol and mixtures thereof. Generally, the solvent system shouldcontain from about 20 to about 75 percent by weight alcohol to ensurethe solubility of the siloxanol formed by condensation of the silanol.If desired, minor amounts, i.e., up to about 20 percent by weight, of awater-miscible polar solvent such as acetone, butyl cellosolve,diacetone alcohol, and the like can be added to the water-alcoholsolvent system. Particularly good results have been observed herein withcompositions containing about 5-10 percent by weight of the totalcomposition of diacetone alcohol. The preferred cosolvent system is amixture of isobutanol and diacetone alcohol.

Generally, sufficient alcohol or water-alcohol solvent is added to givea composition containing from about 10 to about 50 percent by weight ofsolids, said solids generally comprising from about 10 to about 70percent by weight of colloidal silica and from about 30 to about 90percent by weight of the partial condensate of the silanol.

The above-described organopolysiloxane coating compositions also containa silylated ultraviolet radiation screening compound. Suitable suchcompounds are described in the aforementioned Ching applications Ser.Nos. 154,623 and 154,624, incorporated herein by reference. The UVscreening compounds used in accordance with the present invention mustbe soluble in, and otherwise compatible with, the organopolysiloxanehydrolyzate. They copolymerize into the silicone, as demonstrated by thelow volatility upon curing and continuous heating. The UV screeningcompounds used in this invention are particularly effective inprotecting polycarbonate from discoloration when the present coatingcompositions are applied to the surface thereof. Suitable compoundsinclude hindered amine- and modified benzophenone-type UV screens. Apreferred compound is 4(3-triethoxysilylpropoxy)-2-hydroxybenzophenone[SHBP].

Any amount of ultraviolet radiation screening compound which iseffective to prevent discoloration of the substrate to which the UVscreen-containing composition is applied can be used. In general, bestresults are obtained where the UV compound is employed in amounts offrom about 8 percent to about 20 percent by weight of the total solidsof the coating composition. Levels of about 12 percent to about 16percent by weight are most preferred.

Other additives and modifying agents, such as thickeners, pigments, dyesand the like, may also be added to the coating compositions of thepresent invention. A particularly desirable additive has been found tobe a small amount of a polysiloxane polyether copolymer flow controladditive. It has been found that these compounds have a beneficialeffect in preventing the occurrence of undesirable flowmarks anddirtmarks which sometimes occur with the application of the coatingcomposition to a substrate. A particularly useful polysiloxane polyethercopolymer for the purposes herein is known as SF-1066, available fromthe General Electric Company; another is BYK-300, sold by Mallinkrodt.The preparation, further description and structural formulae for thesepolysiloxane polyether copolymers are described in U.S. Pat. Nos.3,629,165 and 4,277,287, incorporated herein by reference. Generally,the polysiloxane polyether copolymers are employed in amounts of fromabout 2.5% to about 15% by weight of the total solids content of thecomposition.

As discussed previously, unless a Lewis acid compound is added to the UVscreen-containing coating composition according to this invention, ithas been found essential that the total admixture be allowed to agebefore use. By aging, it is meant standing at 10° C., or above,preferably at about room temperature, e.g., 18°-24° C., for at least 42hours, but preferably for a minimum time of from about 20 to 30 days.While the nature of aging process with respect to the compositions ofthis invention is not fully understood, it is believed that theultraviolet radiation screening compound is actually being chemicallyincorporated into the structure of the organopolysiloxane hydrolyzate.In any event, it has been found that if the compositions are notproperly aged after the addition of the UV screen, inferior adhesion ofthe coating to the unprimed substrate, cracking, and poorer abrasionresistance may result.

Addition of a Lewis acid compound to the coating compositions of thisinvention effectively reduces the required aging period to about 3 to 7days. Compositions thus treated will still exhibit primerless adhesionto polycarbonate substrates as well as improved thermoformability andother advantages already mentioned. Alternatively, the UVscreen-containing coating compositions, with or without the Lewis acidcompound, can be used with the primer compositions described herein toyield thermoformable, protectively coated thermoplastic articles.

The coating compositions of this invention will cure on polycarbonate attemperatures of, for example, 125° C. without the aid of an added curingcatalyst. However, in general, catalyst in the amounts of from about0.05 to about 0.5 weight percent, preferably about 0.1 weight percent,of the composition can be used. Compositions containing catalysts inthese amounts can be cured on a solid substrate in a relatively shorttime at temperatures in the range of from about 90° C.-200° C. toprovide a transparent, abrasion-resistant surface coating.

The organopolysiloxane coating composition, whether applied as a solecoat or over a primer coat, may be applied by any of the commonly knownmethods such as spraying, dipping, flow-coating, etc. By choice of theproper formulation and processing conditions, a variety of serviceablecoating compositions and coating systems is provided. A hard coatinghaving all of the aforementioned characteristics and advantages isobtained by the removal of the solvent and volatile materials. The topcoating composition will air-dry to a tack-free condition, but heatingin the range of 90° C. to 200° C. is necessary to obtain condensation ofresidual silanols in the partial condensate. To obtain condensationbelow about 125° C., the compositions must be properly catalyzed. Curingat temperatures as low as 75° C. is possible with high activitycondensation catalysts, although this type of catalyst is ordinarilyonly used where long shelf life is not important. This final cureresults in the formation of silsesquioxane (RSiO_(3/2)). In the finishedcure top coating having a ratio of RSiO_(3/2) to SiO₂ will range fromabout 0.43 to about 9.0, preferably 1 to 3. A cured top coating having aratio of RSiO_(3/2) to SiO₂, where R is methyl, of 2 is most preferred.Coating resistance may vary, but for the high abrasion resistancedesired herein, coating thickensses of 3-10 microns, preferably 5microns, are utilized.

In order that those skilled in the art may better understand how topractice the present invention, the following examples are given by wayof illustration and not by way of limitation.

EXAMPLE I

A stock colloidal silica filled thermoset resin coating composition wasprepared as follows:

22.1 parts by weight of Ludox LS® silica sol (aqueous dispersion ofcolloidal silica; Dupont) was added to a solution of 0.08 parts byweight of acetic acid in 26.8 parts by weight of methyltrimethoxysilane.The temperature of the reaction was maintained at 25° C. The hydrolysiswas allowed to continue for about 24 hours. The resulting hydrolyzatewas about 40.6% solids and was diluted to about 20% solids by theaddition of isobutanol. 0.8 parts by weight (4% of solids) of SF-1066(polysiloxane polyether copolymer; General Electric Co.) was thoroughlymixed with 99 parts by weight of the hydrolyzate composition. The finalcomposition had a pH of 7.3.

Four modified resins were made from the stock resin, as follows(proportions are in parts by weight):

    ______________________________________                                                    Compositions                                                                  A    B         C      D                                           ______________________________________                                        stock resin   3300   3300      3300 3300                                      SHBP*          79     79        79   79                                       diacetone alcohol                                                                           --      26       --    26                                       isobutanol     316    290       316  290                                      ferric chloride                                                                             --     --        0.21 0.21                                      ______________________________________                                         *4(3-triethoxysilylpropoxy)-2-hydroxybenzophenone   By blending the           portions of these four resins and adding additional water, isobutanol and     diacetone alcohol, the solids content and combinations of iron, water and     diacetone alcohol were varied. SHBP concentration was maintained at 12% by     weight of the original stock resin solids.

Test coating compositions were coated on Lexan® sheets, dried 20 minutesand cured 5 hours at 135° C. The sheets were then cut into plaques fortesting. All sheets passed an initial scribed adhesion test, wherein agrid of 100 1 mm by 1 mm squares is cut on the surface of a sheet, 3M710 tape applied and pulled off. Three tape pulls without adhesionfailure is considered passing.

The plaques were tested for abrasion resistance, adhesion loss due tomoisture and resistance to ultraviolet radiation weathering. Abrasionresistance was tested on a Taber Abraser in which the increase in haze(Δ%H) is measured after the coated surface is subjected to 500 cycles ofabrasive CF-10 wheels with a 500 g weight on each wheel. Adhesion losswas tested by immersing plaques in a 65° C. water bath and periodicallyremoving plaques for scribed adhesion testing, described above, untilcoating adhesion failed. UV- and moisture resistance were tested on aQUV Accelerated Weathering Tester, in which plaques are exposed toalternating cycles of UV radiation at 60° C. for 8 hours, followed by acondensation cycle at 50° C. for 4 hours. Plaques are periodicallyremoved and subjected to scribed adhesion testing until coating adhesionfails.

The results of these tests are shown in the following tables. "Top","Middle" and "Bottom" refers to the section of the coated Lexan® sheetsfrom which the plaques were cut.

    ______________________________________                                        Test                       % Diacetone                                        Compositions                                                                           % Solids % Water  Alcohol  δ ppm Iron                          ______________________________________                                        1        20        8.6     0.8      10                                        2        20       11.1     0.8      20                                        3        20       11.0     0.8      0                                         4        20       11.0     1.6      10                                        5        20       11.0     0.0      10                                        6        20       13.6     0.8      10                                        7        18       10.8     0.8      10                                        8        18       11.0     0.8      10                                        9        18       11.2     0.0      20                                        10       18       11.0     0.0      0                                         11       18        8.4     1.6      10                                        12       18        8.5     0.0      10                                        13       18        8.6     0.8      20.0                                      14       18        8.6     0.8      0.0                                       15       18       11.0     0.8      10.0                                      16       18       11.1     1.6      20.0                                      17       18       11.2     1.6      0.0                                       18       18       13.4     0.0      10.0                                      19       18       13.4     0.8      20.0                                      20       18       13.5     0.8      0.0                                       21       18       13.3     1.6      10.0                                      22       16        8.4     0.8      10.0                                      23       16       10.6     0.0      10.0                                      24       16       10.7     0.8      20.0                                      25       16       10.6     0.8      0.0                                       26       16       10.7     1.6      10.0                                      27       16       13.0     0.8      10.0                                      ______________________________________                                    

                  TABLE I                                                         ______________________________________                                        ABRASION RESISTANCE (Δ % H.sub.500)                                     Test                                                                          Composition Average  Top      Middle                                                                              Bottom                                    ______________________________________                                        1           4.5      7.6      4.6   4.4                                       2           5.8      8.0      3.9   5.6                                       3           7.2      11.5     4.4   5.6                                       4           6.3      9.0      4.1   5.8                                       5           6.1      7.0      5.0   6.3                                       6           5.8      6.7      5.1   5.6                                       7           8.6      14.3     5.1   6.3                                       8           6.6      9.8      4.1   5.8                                       9           6.9      10.7     4.1   5.9                                       10          6.9      10.2     4.4   6.1                                       11          7.1      11.5     4.0   5.8                                       12          6.7      9.6      4.9   5.6                                       13          6.2      8.7      4.4   5.5                                       14          6.8      7.9      5.2   7.2                                       15          7.3      9.4      5.3   7.2                                       16          6.6      9.1      4.4   6.4                                       17          6.9      10.0     5.4   5.2                                       18          6.5      8.4      4.7   6.3                                       19          6.1      8.2      5.0   5.1                                       20          7.3      9.3      5.4   7.1                                       21          6.4      8.8      4.8   5.5                                       22          7.6      11.7     5.5   5.6                                       23          9.3      16.2     5.4   6.3                                       24          11.6     24.4     5.4   4.9                                       25          10.2     16.3     6.5   7.8                                       26          9.9      14.5     6.3   8.9                                       27          8.8      15.2     5.1   6.0                                       ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        65° C. WATER SOAK (hours to adhesion failure)                          Test                                                                          Composition Average  Top       Middle                                                                              Bottom                                   ______________________________________                                        1           144      144       168   120                                      2           216      216       216   216                                      3           120      120       144   96                                       4           176      192       168   168                                      5           88       96        72    96                                       6           96       96        96    96                                       7           120      120       144   96                                       8           144      168       144   120                                      9           128      120       144   120                                      10          96       120       96    72                                       11          120      144       144   72                                       12          96       120       96    72                                       13          112      120       144   72                                       14          72       72        72    72                                       15          72       72        72    72                                       16          176      216       216   96                                       17          136      168       144   96                                       18          96       96        96    96                                       19          104      96        96    120                                      20          80       72        72    96                                       21          120      120       144   96                                       22          120      120       144   96                                       23          65       72        72    52                                       24          72       72        72    72                                       25          156      120       192   --                                       26          65       72        72    52                                       27          80       96        72    72                                       ______________________________________                                    

                  TABLE III                                                       ______________________________________                                        QUV HOURS (hours to adhesion failure)                                         Test                                                                          Composition                                                                             Average   Top     Middle  Bottom                                    ______________________________________                                        1         1316      598     1195    2155                                      2         1084      598     1098    1555                                      3         1012      526     1195    1316                                      4         1179      598     1267    1673                                      5         739       428     741     1048                                      6         988       526     1027    1410                                      7         717       526     741     884                                       8         924       428     1027    1316                                      9         636       428     667     813                                       10        652       574     860     523                                       11        924       428     1027    1316                                      12        709       574     741     813                                       13        948       500     1027    1316                                      14        660       500     667     813                                       15        628       500     667     716                                       16        971       500     1098    1316                                      17        892       500     860     1316                                      18        708       500     741     884                                       19        749       500     933     813                                       20        892       500     860     1316                                      21        780       500     860     981                                       22        628       500     667     716                                       23        484       428     500     523                                       24        524       428     500     643                                       25        644       428     860     813                                       26        789       526     860     981                                       27        636       428     667     813                                       ______________________________________                                    

The thermoformability performance of the test compositions was alsotested. Lexan® sheets coated with a given composition were cold formedon a jig to a curved shape having a 32" radius. The curved sheets werethen thermoformed by placing them in an oven at 138° C. for 1 hour. Thecurved sheets were then removed from the oven, cooled, removed from thejig, and examined for cracks.

The thermoformability results observed are set forth in the followingtable ("incipient cracks" are cracks that occur only along the cutedge):

    ______________________________________                                        Test              Cracks Observed                                             Composition       After Thermoforming                                         ______________________________________                                        1                 bad                                                         2                 moderate                                                    3                 bad                                                         4                 bad                                                         5                 bad                                                         6                 bad                                                         7                 moderate                                                    8                 slight                                                      9                 moderate                                                    10                moderate                                                    11                moderate                                                    12                slight                                                      13                slight                                                      14                moderate                                                    15                bad                                                         16                moderate                                                    17                bad                                                         18                slight                                                      19                moderate                                                    20                bad                                                         21                moderate                                                    22                very slight                                                 23                very slight                                                 24                incipient                                                   25                slight                                                      26                slight                                                      27                incipient                                                   ______________________________________                                    

EXAMPLE II

A colloidal silica filled thermoset organopolysiloxane coatingcomposition containing 12% by weight of solids SHBP was prepared as inExample I.

A set of primer compositions was prepared by dissolving poly(ethylmethacrylate) resin (Elvacite®2042; Dupont) in 2-butoxyethanol at 1/2, 1and 2% solids by weight. Each of the primers was flow coated on Lexan®polycarbonate plaques, allowed to air dry for 15 minutes, and thermallydried for 20 minutes at 125° C. The plaques were then flow coated withthe coating composition, which had been previously diluted to 20% solidsand aged several weeks at room temperature to hydrolyze the SHBP. Thecoated plaques were dried at room temperature for 30 minutes, then cured45 minutes at 130° C.

The plaques were cold-formed on a jig to a 32-inch radius, then heatedfor 1 hour at 138° C. The plaques were removed from the oven, allowed tocool, removed from the jig, and inspected for cracks. For all 3 primercompositions, only incipient edge cracks were observed in the formedproduct.

Similar results were obtained with thermally dried poly(isobutylmethacrylate) primer compositions. Plaques coated with non-extensiblepolymethyl methacrylate primers (1/2 and 1% solids by weight) showedsevere cracking when thermoformed. Plaques coated with theSHBP-containing coating composition but without a primer werethermoformable but showed weaker weathering performance (see Control,infra).

    ______________________________________                                                    QUV Exposure:                                                                                Adhesion                                                                              H.sub.2 O Soak Until                       Sample   Δ % H.sub.500                                                                    Cracking Failure Adhesion Loss                              ______________________________________                                        1/2% PEMA*                                                                             9.6      326 hours                                                                              1021 hours                                                                            216 hours                                  Primer                                                                        1.0%     9.4      326 hours                                                                               877 hours                                                                            336 hours                                  2.0%     9.9      mottled  1286 hours                                                                            336 hours                                                    at                                                                            326 hours                                                   Control**         183 hours                                                                               877 hours                                                                             65 hours                                  ______________________________________                                         *poly(ethyl methacrylate)                                                     **coating composition applied without primer                             

Obviously, modifications and variations in the present invention arepossible in light of the foregoing disclosure. It is understood,however, that any incidental changes made in the particular embodimentsof the invention as disclosed are within the full intended scope of theinvention as defined by the appended claims.

I claim:
 1. A thermoformable silicone resin coating compositioncomprising a colloidal silica filled thermoset organopolysiloxanecontaining a silylated ultraviolet radiation screening compound and asmall amount of a Lewis acid compound.
 2. A composition as defined inclaim 1, wherein said Lewis acid compound is selected from the groupconsisting of ferric chloride, aluminum chloride and zinc chloride.
 3. Acomposition as defined in claim 2, wherein said Lewis acid compound isferric chloride.
 4. A composition as defined in claim 3 which containssufficient ferric chloride to provide 10 to 30 ppm iron.
 5. Acomposition as defined in claim 1 which also contains a small amount ofa polysiloxane polyether copolymer.
 6. A method for providing athermoformable coating on polycarbonate substrates which comprises thesteps:(A) applying to a polycarbonate substrate an extensible primercomposition comprising from about 0.5 to about 10 parts by weight of athermoplastic acrylic polymer in about 99.5 to about 90 parts by weightof a volatile solvent, and thereafter (B) applying a thermoformablesilicone resin coating composition as defined in claim
 1. 7. A method asdefined in claim 6, wherein said volatile solvent is selected from thegroup consisting of ethyleneglycol diacetate, butoxyethanol, ethoxyethanol, diacetone alcohol, hydroxyethyl acetate, cellosolve acetate,and combinations thereof.
 8. A method as defined in claim 7, whereinsaid acrylic polymer is poly(ethyl methacrylate).
 9. A method as definedin claim 7, wherein said acrylic polymer is poly(isobutyl methacrylate).10. A method as defined in claim 6, wherein the Lewis acid compound isferric chloride.
 11. A method as defined in claim 7, wherein the acrylicpolymer is poly(ethyl methacrylate), the volatile solvent is2-butoxyethanol and the Lewis acid compound is ferric chloride.
 12. Amethod for providing a thermoformable coating on polycarbonatesubstrates which comprises applying to a polycarbonate substrate athermoformable silicone resin coating composition as defined in claim 1.13. A method as defined in claim 12, wherein said Lewis acid compound isselected from the group consisting of ferric chloride, aluminum chlorideand zinc chloride.
 14. A method as defined in claim 13, wherein saidLewis acid compound is ferric chloride.
 15. A method as defined in claim14, wherein said ferric chloride is present in an amount to provide 10to 30 ppm iron.